Fix typo in README

Thanks to @ikotler for spotting it.

Closes: #36

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

README.md

@@ -108,7 +108,7 @@ def handle(req):
     """
 
     # req is bytes, so an input of "hello" returns i.e. b'hello'
-    return string(req)
+    return str(req)
 ```
 
 # Using the python3-http templates
@@ -241,4 +241,4 @@ def handle(event, context):
             "content-type-received": event.headers['Content-Type']
         }
     }
-```
+```